This application relates to the field of producing an improved spinal prosthesis that is used in surgical procedures, and more particularly to a minimally invasive inter-vertebra expandable cage to replace dysfunctional spinal discs.
Methods to achieve the fusion of two adjacent vertebra-bodies are well known in the art. In these procedures, it is necessary to maintain (or regain) the original distance between the vertebra bodies, as otherwise problems, such as nerve damage, may occur. Therefore, it is important to pull or push the vertebra bodies apart during surgery and to place a support that is strong enough to withstand the axial compression forces during normal use. In a human body, these forces can be as high as 1000 N. In conventional spinal fusion procedures, the area of the spine that has to be operated on is approached from the front side, where relatively large incisions have to be made. An example of such a procedure is described in xe2x80x9cSubtotal and total vertebral body replacement and interbody fusion with porous Tixe2x80x94Ni implantsxe2x80x9d, by B. Silberstein in proceedings of SMST-97, Pacific Grove, Calif., USA, on pages 617-621. In this procedure, a rigid porous metal implant is placed between the vertebra bodies, while a tension force is put onto the spine. This method requires a relatively large operation place around the area where the implant must be inserted.
Another method was developed by Krupp Medizintechnik in Essen, Germany, where an expandable ring was made of a nickelxe2x80x94titanium (NiTi) shape memory alloy. This ring was applied in a compressed shape in the inter-vertebra gap and, after insertion, was heated to recover to the programmed height, which resulted in an expansion dimension twice as large as in the compressed state. After placement and expansion, this hollow ring was filled with bone particles and, after several weeks, total fusion was achieved. However, the initial stability of such a ring, in combination with loose bone particles, was very restricted. This necessitated that the patient had to stay relatively immobile for a long time. (See, for example, G. Bensmann et al. in xe2x80x9cxc3x9cfntersuchungsberichte Krupp Forschungsinstitutxe2x80x9d, Band 42, 1984, pages 22-38).
This invention relates to an improved prosthetic device for a minimally invasive permanent spinal fusion that doesn""t suffer the limitations of the prior art. In the present context, a spinal fusion procedure is considered xe2x80x9cpermanentxe2x80x9d when the formation of a rigid bone graft between adjacent vertebrae is effected such that it restores spinal functionality to a level approaching that prior to the injury, disorder or disease that necessitated the procedure.
According to an aspect of the present invention, an expandable intervertebral prosthesis for replacement of the nucleus of an intervertebral disc is disclosed. The prosthesis comprises at least one cage made from a relatively thin walled material, and a reinforcing element disposed within a substantially hollow central region of the cage created when the cage is expanded. As used in conjunction with the present disclosure, the term xe2x80x9csubstantiallyxe2x80x9d refers to an arrangement of elements or features that, while in theory would be expected to exhibit exact correspondence or behavior, may in practice embody something slightly less than exact. The reinforcing element rolls up from an oblong geometry into at least one substantially planar spiral geometry in a force-free state. The reinforcing element occupies the central region of the expanded cage in such a way that stability along a substantially longitudinal axis of the cage is increased, thereby enabling a permanent spinal fusion. In the present context, the longitudinal axis of the cage is substantially aligned with the lengthwise axis of the spine. Accordingly, the vertebrae stack together along the longitudinal, or Z, axis. This axis also defines a longitudinal axis of the intervertebral space.
The reinforcing element may initially have an oblong geometry resembling a strip, which can be of straight or slightly curved cross-section. Upon deployment in the substantially hollow central region of the expanded cage, the oblong shape transforms into a compact body in the shape of a substantially planar spiral. This spiral fills the central region of the cage in such a way that axial stability is increased considerably, while still enabling a permanent spinal fusion. Material choices in both the reinforcing element and the cage may further enhance the utility of the prosthesis. For example, the reinforcing element can be made of any material and form that gives it the capability to be delivered in its initial oblong shape, and then rolled up into its spiral shape in the expanded cage. Examples may include surgical-grade steel, or a shape memory material with superelastic or shape memory behavior. Similarly, the cage can be made of any material and form that give it the capability to be delivered in an collapsed shape and to expand when it is in place. It too can be made from any biocompatible polymer or metal, but preferably of shape memory material with superelastic or shape memory behavior, with their inherently high strain-to-failure ratio. The cage can also be heat-treated to give it the tendency to open up at body temperature by its superelastic properties, and therefore elastic energy will be stored in the cage as long as it is collapsed. In the latter case, the material can be heated above its transformation temperature by means of body heat or any additional heating source. By use of a shape memory cage, the separate restraining tool may not be necessary.
Hinges may be disposed at various locations within the cage to provide preferential bending locations, and thereby facilitate the cage""s expansion once it has been deployed in an intervertebral position. These hinges, capable of elastic or plastic deformation, enable both a large expansion ratio and well-defined final cage geometry after expansion. The hinged locations may be formed by any number of conventional methods, including, but not limited to, cutting, grinding, scoring or heat treatment. The hinges may, in the alternate, be conventional pinned hinge members. In addition, the hinges may be configured with mechanical stop or locking features so that once expanded, the cage may settle in to a final expanded state, providing additional longitudinal direction stability and strength. Similarly, one or more apertures may be included in the one or more surfaces of the cage, and may additionally disposed adjacent the hinges. These apertures permit the insertion of an elongate instrument into the unexpanded cage to keep the cage unexpanded until such time as the elongate instrument is released. The elongate instrument comprises an outer tube and a tension wire disposed within the outer tube so that relative movement along the elongate instrument""s elongate axis is permitted. To avoid the rapid expansion of the cage, the elongate instrument includes an additional restraining element disposed on the tension wire. This restraining element engages complementary apertures placed within the cage, and keeps the cage in its unexpanded state, even after the cage has left the distal end of the delivery tube. Once the cage is placed in the desired location, the restraining element is gradually released and retracted back into the delivery tube, thus enabling the full expansion of the cage. The unexpanded cage, reinforcing element and elongate instrument can be used with conventional delivery apparatus, such as a delivery tube, to deliver them into the intervertebral space prior to expansion and reinforcement.
One or more of the cages may be combined once they are deployed in the intervertebral space, leading to the formation of an assembled cage. After placement of a first cage, a second cage can be placed in a different position or orientation to construct an assembled cage that provides more support to the vertebrae and gives stability along more directions. Mating surfaces between the first and second cage may include interlocking features, such as grooves, recessions or protrusions to ensure a proper relative placement between the two. The substantially hollow central region within the assembled cage can be defined by the space between the two interconnected cages such that it is bounded by top and bottom walls, and partially bounded by side walls.
According to another aspect of the invention, an expandable intervertebral prosthesis with increased axial stability in a direction substantially parallel with a longitudinal spinal axis is disclosed. The prosthesis includes a first cage in an unexpanded state, a second cage placed inside the first expanded state cage, and a reinforcing element that rolls up from an oblong geometry into at least one substantially planar spiral geometry in a force-free state. The cages are adapted to be placed in an intervertebral position within a spine, and then caused to expand to one or more expanded states. The cages are interconnected such that when both cages are in at least one of their expanded states, they together form an assembled cage, which, in turn defines a substantially hollow central region. The reinforcing element at least partly fills the substantially hollow central region in the assembled cage, thereby forming a reinforced cage.
Optionally, the reinforcing element forms a form-fitted spiral without intervening spaces in the force-free state. In addition, the form-fitted spiral has axial surfaces in substantial contact with corresponding inner surfaces of the cage assembly to enable the cage assembly to take up high axial loads. Moreover, the reinforced cage further comprises sterilizable material with deformation properties corresponding essentially to those of an intervertebral disc to be replaced.
According to another aspect of the invention, an expandable prosthesis for replacement of an intervertebral disc to promote spinal fusion is disclosed. The prosthesis includes at least one cage made from a relatively thin-walled material, and a reinforcing element disposed within a substantially hollow central region defined by at least one the cage. The at least one cage comprises a first, unexpanded state prior to deployment into an intervertebral space, and a second state wherein after deployment in the intervertebral space, the at least one cage is expanded. The reinforcing element comprises a first, oblong state prior to deployment into the substantially hollow central region, and a second state wherein it assumes a substantially planar spiral configuration upon deployment in the substantially hollow central region. The substantially planar spiral configuration at least partly fills the substantially hollow central region in such a way as to improve the stability of the expanded at least one cage in a direction substantially parallel to a longitudinal axis of the intervertebral space.
According to another aspect of the invention, a prosthesis expandable in a plurality of dimensions to promote intervertebral fusion is disclosed. The prosthesis includes a cage and a reinforcing element. The cage further includes a first, unexpanded state prior to deployment into an intervertebral space and a second, expanded state. The second, expanded state itself includes a first expansion in a direction substantially parallel to a longitudinal axis of the intervertebral space thereby defining a substantially hollow central region, and a second expansion in a plane normal to the first expansion such that a space formed by the first and second expansions defines a substantially hollow central region. The reinforcing element includes a first, oblong state prior to deployment into the substantially hollow central region, and a second state wherein the reinforcing element assumes a substantially planar spiral configuration upon deployment in the substantially hollow central region. The substantially planar spiral configuration assumed by the reinforcing element in its second state at least partly fills the substantially hollow central region in such a way as to improve the stability of the expanded cage in a direction substantially parallel to the longitudinal axis of the intervertebral space. Optionally, the cage also includes a plurality of hinges, at least some of which are designed to pivot around an axis substantially parallel to the longitudinal axis of the intervertebral space.
According to yet another embodiment of the invention, a method of using an intervertebral prosthesis is disclosed. The method includes the steps of providing a prosthesis, spreading apart adjacent vertebrae such that an intervertebral space is defined thereby, placing the prosthesis in a delivery tube, inserting the delivery tube into the intervertebral space, controlling the expansion of at least one expandable cage of the prosthesis through limiting the relative movement between the outer tube and tension wire, using the elongate instrument, which is adapted to control the expansion of the at least one expandable cage, to remove the at least one expandable cage from the delivery tube, placing the at least one expandable cage in the intervertebral space, expanding the at least one expandable cage, and inserting a reinforcing element into a substantially hollow central region defined by the expansion of the at least one expandable cage such that the reinforcing element rolls up from an oblong geometry into at least one substantially planar spiral geometry in a force-free state that at least partly fills the substantially hollow central region. The elongate instrument further comprises an outer tube, a tension wire disposed within and relatively movable to the outer tube and a restraining element disposed on the tension wire. The restraining element is further adapted to engage at least a portion of the at least one expandable cage.
It is contemplated that variations in structural features and arrangement of parts may appear to a person skilled in the art without departing from the scope of or sacrificing any of the advantages of the invention. Accordingly, other features and advantages of the invention will be apparent from the following description, the accompanying drawings and appended claims.